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Colias croceus
I estimate the wingspan to be +/- 4cm.
Prefers to spend 96% of its time just out of camera range. This one was flitting about a manicured lawn going from clover to clover while I stalked it like a escaped mental patient. I apologize for the drama, but...gug. From Butterfly Conservation (link to right): The Clouded Yellow is one of the truly migratory European butterflies and a regular visitor to Britain and Ireland. Although some of these golden-yellow butterflies are seen every year, the species is famous for occasional mass immigrations and subsequent breeding, which are fondly and long remembered as ''Clouded Yellow Years''. A small proportion of females are pale yellow (form helice), which can be confused with the rarer Pale and Berger's Clouded Yellows.
C. croceus might not be the exact species.
You are welcome. It is just a humble hint as for the species I am no expert and may point you at the wrong one ;-)